Draft 

STRATEGIES FOR VALIDATING THE BLOCKSON RADON MODEL 

AND ADDRESSING STRATIFICATION ISSUES 

Prepared by 

John Mauro, SC&A, Inc. 

1608 Spring Hill Road, Suite 400 

Vienna, Virginia 22182 

Revised September 9, 2009 
Blockson White Paper 2 SC&A – Revised September 9, 2009 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

INTRODUCTION 

During the full Advisory Board meeting held in West Chester, Ohio, on July 27–29, 2009, there 

was some discussion regarding the radon model developed for the purpose of placing a plausible 

upper bound on the average annual radon concentrations that workers in Building 40 might have 

experienced during the covered period. Following those discussions, the Advisory Board voted 

on a motion to accept the model. The Board’s vote was evenly divided with regard to this 

motion, thereby effectively resulting in non-acceptance of the model. It is SC&A’s 

understanding that this was an important vote, because without an accepted method for 

addressing the radon issue, it might not be possible to move forward on the larger issue related to 

the Special Exposure Cohort (SEC) petition for the Blockson Chemical Company. 

Based on the Board’s discussions, it is SC&A’s understanding that the primary issues related to 

the acceptance of the model are related to the fact that there are no radon measurements available 

for Building 40 for the time period of interest that could be used to validate the model, nor were 

any data or reports provided from other venues that might be useful in validating the model. In 

a related matter, one or more members of the Board also expressed concern that “stratification” 

was not explicitly taken into consideration in the model. The stratification issue has to do with 

the possibility that there might be locations within the building where the average annual radon 

concentrations were higher than the average annual radon concentrations derived for the overall 

building, as derived using the model. If this were, in fact, the case, then workers who might have 

occupied those locations could have experienced annual exposures to radon that were higher than 

those derived by the model. 

SC&A would like to point out that, notwithstanding differences of opinion regarding the input 

parameter distributions employed in the model, the model was used by both NIOSH and SC&A 

to derive the upper 95th percentile concentration of the average annual radon concentration in 

the building. In performing these calculations, stratification was not explicitly taken into 

consideration. However, heuristic arguments were made by both SC&A and NIOSH that, given 

our understanding that workers were not at fixed locations, air flow patterns were complex and 

ever changing, and given that the upper 95th percentile concentration was selected for performing 

dose reconstructions, stratification is unlikely to have a significant impact on the dose 

reconstructions of individual workers. However, SC&A acknowledges that this perspective 

regarding the stratification issue is certainly a subjective judgment. 

In order to help address issues related to model validation and stratification, the Board requested 

that both SC&A and NIOSH give some thought to lines of inquiry that might help to resolve 

these concerns. This report is provided in response to that request. 

When addressing questions such as these, it is sometimes helpful to ask, What information would 

answer these questions? Under ideal circumstances, we would have access to data gathered 

from continuous radon monitors arranged in a 10 × 10 × 10 meter, 3-dimensional grid distributed 

over the entire volume of the building. Each location would be representative of the average 

radon concentration in a volume of 1,000 m3. In this way, we would be able to estimate the 

average annual concentration of radon in each 1,000 m3

 volume within the building (which we 
Blockson White Paper 3 SC&A – Revised September 9, 2009 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

estimate was between 17,321 and 23,983 m3). We would then make some judgment regarding 

how much time each worker spent in each location in the building. Under worst-case conditions, 

we could assume workers spend all their time in the 1,000 m3

 volume with the highest average 

annual radon concentration. However, we don’t have this information. If we did, we wouldn’t 

need the model, but such data would be extremely useful in validating the model for use at other 

applications. 

STRATIFICATION 

There is little doubt that the radon concentration varied a great deal as a function of time and 

location within Building 40. The question is, however, did the average annual radon 

concentration at a given location differ significantly from the overall average radon 

concentration in the building? Given that such a gradient existed, and assuming that some 

workers occupied those locations for protracted periods of time, is it possible that the upper 95th

percentile of the derived average annual concentration for the entire building would 

underestimate the annual radon exposures to some workers? We do not believe it is possible to 

provide a hard and fast answer to this question. However, there are certain lines of inquiry that 

might help to inform such judgments. 

Strategy No. 1 

One approach that might be used to address this question is to take advantage of what we know 

about Building 40. We believe that the most likely location from which radon would have been 

emitted into the atmosphere of Building 40 was at the surface of the digester tanks. We believe 

that when the crushed ore concentrate was dissolved in the sulfuric acid in the digesters, radon 

was also dissolved in the acid. The acid was stirred and was hot, and the digesters were open at 

the top. Hence, we believe that most of the radon that entered the atmosphere of Building 40 

emerged from the surface of the digester tanks. 

It is our understanding that these tanks were located on the upper level of the building and ran 

along the length of the building. Hence, it is likely that the location with the highest average 

annual radon concentration was the volume of air between the top surface of the digester tanks 

and the roof of the building. Given this conceptual model, one could calculate the concentration 

of radon in the air above the tanks as it rose vertically from the surface of the digesters and 

moved up toward the ceiling, where it exited the building via roof fans and/or vents. The rate at 

which the air left the building via this route can be estimated by using information that has been 

already developed regarding the air turnover rate for the building, and converting that rate 

(which is expressed in units of 1/hr) into a flow rate in units of m3

/hr. This can be accomplished 

by estimating the surface area of the ceiling above the digesters. Hence, atoms/sec of radon 

leaving the surface of the digester divided by cubic meters per second of air leaving the building 

via the roof vents yields atoms per cubic meter in the volume of air above the digester tanks. 

This would seem to be one simple and conservative method for assigning an upper bound to the 

average annual radon concentration within a segment of Building 40. 
Blockson White Paper 4 SC&A – Revised September 9, 2009 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

Strategy No. 2 

One possible approach that can be used to explicitly address stratification is to assume that the 

concentration of radon in Building 40 has a 3-dimensional Gaussian distribution (i.e., has the 

profile of a normal distribution curve). In the simplest version of such an approach, it can be 

assumed that the distribution is spherically symmetrical. The average concentration of radon in 

the building can be assumed to be the same as that calculated by the existing model. However, 

the concentration at the building’s boundary can be assumed to equal the outdoor ambient 

concentration of radon, i.e., the environmental background concentration. This assumption 

should place an upper bound on the concentration gradient in the building. When inserted into 

the existing Monte Carlo model, this will have no effect on the geometric mean, but will increase 

the spread of the calculated values. Such a calculation would produce an estimate of the 

maximum plausible value of the 95th percentile of the average annual concentration of radon 

anywhere in the building, which explicitly takes stratification into consideration. Since it is 

implausible that the concentration at the periphery of the building would be less than the outdoor 

concentration, we believe that the actual gradient cannot be steeper than the one derived by this 

procedure. 

Strategy No. 3 

Although there are limited data on radon levels in phosphate production plants, it may be 

possible to assemble data on measurements taken inside a given facility within the same 

timeframe. These measurements could be used to construct a normalized frequency distribution. 

This distribution, which would be assigned a geometric mean of 1, would characterize the 

variability of radon levels at different locations. It would be a simple matter to include such a 

distribution in the current radon model. The geometric mean of the concentrations calculated by 

the Monte Carlo model would not change; however, the distribution of results would now 

explicitly include the gradient based on the variability of the actual measurements. The revised 

model would thus encompass the relative variability of the measurements, not the actual values 

of these measurements. It would combine the modeled concentrations, based on the specific 

characteristics of Building 40, with the gradient found in actual facilities. Because of the 

additional variable, the resulting concentrations would span a greater range, so the 95th percentile 

value would be somewhat larger than the values derived by SC&A and NIOSH. 

This strategy has the advantage of being based on industrial facilities processing phosphate ore, 

and would better represent the concentration gradients than data on homes, offices, or other 

buildings where the radon enters from the surrounding soil (which is discussed next); the 

difficultly is finding data for facilities that can be reasonable surrogates for Building 40. 
Blockson White Paper 5 SC&A – Revised September 9, 2009 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

 

Strategy No. 4 

There is a vast amount of data and literature on the variability of the concentration of radon in 

homes as a function of season and location within residences. One study1

 revealed that there is a 

several-fold difference in the radon concentration in residences between seasons and between the 

basement and the first and second floors in residences. 

In addition, the EPA published a report in 1993 titled, National Residential Radon Survey. This 

was a survey of 5,600 residences in the United States, where short-term measurements of the 

radon concentrations were made in the basement, first floor, and second floor of each residence. 

The data reveal that the radon concentrations were 2 to 3 times higher in the basement than in the 

first and second floors. Similar data, but not as extensive, has been compiled for schools and 

large structures, such as apartment houses. These data were complied before a nationwide effort 

was implemented to reduce the concentration of radon in homes by the use of subslab ventilation 

systems and other construction techniques designed to mitigate the radon problem in the United 

States. 

In the case of residences and other structures investigated by the EPA, the radon enters the 

structure primarily2

 through the floors and walls of the basement, and then moves vertically 

upward due to the stack/chimney effect, which is caused by the buoyancy of the warm indoor air 

compared with the cold (in winter) outdoor air. This effect is responsible for two mechanisms 

that inject and distribute radon; (1) the production of small, but persistent, air pressure 

differences across the foundation surfaces (that draw soil gas and radon into the building), and 

(2) the flow of outdoor air entering cracks and openings low on the building envelope, passing 

up through the building, and leaving through cracks and openings high on the building envelope. 

In addition, wind effects can also increase or reduce the extant air pressure differences, causing 

greater or lesser amounts of radon entry and altering air movement into and within the building. 

Hence, stratification resulting in a 2- to 3-fold difference in the average annual concentration of 

radon in different levels of residences is well established. 

Certainly the physical setting and the source of the radon in Building 40 is different than in 

residences. In Building 40, the “radon pump” is the radon leaving the surface of the digester 

tanks located on the upper level of the building and moving up with the thermal plume created 

by the hot sulfuric acid. In a home, the radon pump is the radon that is entering the building 

from the basement due to cracks in the concrete floor and the pressure differential between the 

basement, and the radon in the pore space in the soil beneath and surrounding the basement. In 

addition, the different levels in residences are separated by the ceiling above each level in a 

home. We do not believe that these barriers are present in Building 40. In addition, if the radon 

moved relatively quickly from the surface of the digester tanks out of the building, there would 

1

 Andreas C. George and Lawrence E. Hinchliffe, “Measurements of Radon Concentrations in Residential 

Buildings in the Eastern United States,” in Radon and Its Decay Products, Occurrence, Properties, and Health 

Effects, Edited by Philip K. Hopke, ACS Symposium Series 331. American Chemical Society, Washington, DC, 

1987. 2

 Radon also enters residences from ground water pumped into homes for domestic use, and from the 

surfaces of radium-bearing construction materials inside the building. However, these sources of radon are much 

smaller contributors to radon indoors than radon in the soil in the vicinity of the residences. 
Blockson White Paper 6 SC&A – Revised September 9, 2009 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

be little time for progeny in-growth. Reasonable arguments can be made that the gradient in 

occupied areas of a residence might be greater or less than that which might have existed in 

Building 40. Nevertheless, it might be possible to incorporate terms into the radon model that 

explicitly address the possibility of a gradient and use the radon data summarized above as the 

basis to assign a distribution to the gradient. 

MODEL VALIDATION 

Strategy 5 

Investigate existing contaminant transport and dispersion models, such as the following: 

• The applicability of models developed at national laboratories [Lawrence Berkeley National 

Laboratory (LBNL)] to study transport and mixing of contaminants within buildings (see 

examples – http://eetd.lbl.gov/IED/4.html). These studies were conducted in response to the 

potential threat of airborne biological and chemical attacks on a variety of U.S. buildings. 

They investigated the spread and dispersion of pollutants, and the identification of pollutant 

source locations. A review of the literature and discussions with the authors and scientists 

might indicate whether these models could be used to predict contaminant movement and 

accumulation in Building 40. 

• The multi-zone airflow and contaminant transport analysis model, developed by the Indoor 

Air Quality and Ventilation Group at the National Institute of Standards and Technology 

(NIST) – CONTAM 2.4c (http://www.bfrl.nist.gov/IAQanalysis/). While the model is 

intended for multi-zone commercial and residential buildings, it may be possible to adapt it 

for the industrial conditions present in Building 40.